Calcium release from intracellular stores evoked by extracellular ATP in a Xenopus renal epithelial cell line

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Abstract
1 The signal transduction mechanism mediating extracellular adenosine 5′‐triphosphate (ATP)‐induced calcium release in a renal epithelial cell line (A6) was investigated using the whole‐cell voltage‐clamp technique and fura‐2 fluorescence measurement. 2 ATP (10 μM) activated calcium‐dependent non‐selective cation channels in cells held under voltage clamp. 3 Guanosine 5′‐O‐(2‐thiodiphosphate) (GDPβS; 0.1–1.0 mM) in the pipette inhibited the ATP‐activated calcium‐dependent currents. With guanosine 5′‐O‐(3‐thiotriphosphate) (GTPγS; 0.1–1.0 mM) in the pipette, the currents were spontaneously elicited without application of ATP. Pretreatment with pertussis toxin (PTX) affected neither the ATP‐activated currents nor the increase in intracellular free calcium concentration ([Ca 2+])i evoked by ATP. 4 Intracellular application of neomycin or heparin inhibited the ATP‐activated currents. Inositol 1,4,5‐trisphosphate (IP3; 0.1–100μM) in the internal solution produced currents similar to those due to ATP activation. 5 These results suggest that a PTX ‐insensitive guanosine 5′‐triphosphate (GTP)‐binding regulatory protein (G protein) is involved in extracellular ATP‐induced phosphoinositide turnover and subsequent calcium release from IP3 ‐sensitive stores, which subsequently activates the calcium ‐dependent channels in A6 cells.